He is Nevarran ambassador. If he returns back Nevarra will have chance to start war with Orlais, there are already Civil War.
Ambassadors are very vulnerable at foreign courts. They are outsiders, even among other ambassadors, they can be recalled or expelled on a whim, and if that happens they lose their proximity to court, their job, their access to, say, Countesses who they fancy, and their political influence. Our man Niccolo M was ambassador to Spain for a time; when war broke out, he was blamed, recalled and tortured. If Vivienne has the ear of the Empress, no mere foreign ambassador will gainsay her. Unless they are insane.
Telling from the first chapter of The Masked Empire, blatant insults are very common in the Game. Gaspard's insults weren't very complicated, I must say. From what I could tell, she was layering her insults and threats with niceties and utmost politeness.
Well, I saw the insults and the threats, but I didn't see much politeness beyond a few pretty turns of phrase. You don't call an ambassador "darling" unless you are addressing them from an adjacent pillow. You certainly don't take it upon yourself to list their shortcomings. It was ham-fisted, and as I say, if it were made public, it would reflect badly on her rather than on the person she was writing to.
If she'd had a minion visit this chap, and say the things she'd written, that would be different; no one could trace it to her.
ETA: Is it possible she's trying to goad him into something she can hang him with?
Fererlden is weak land and they have cold war. Nevarra is strong and dangerous.
Orlais is Orlais, though. And even as strong as they are, I suspect the Nevarrans have little to gain by war with Orlais. Not to mention, as above, what the ambassador would stand to lose personally.